Clinical trials designed to evaluate the use of fast neutrons in the radiotherapy of malignant disease are now in progress in this country. The early clinical results appear promising, but at the same time, seem to point out the need for more information about the biological effects of neutrons at the cellular and molecular levels. Data concerning the molecular effects of neutrons are of particular importance since, according to current concepts, DNA is the target primarily responsible for the loss of cellular proliferative capacity. Unfortunately, very few data are currently available regarding the effects of fast neutrons on the DNA of mammalian cells. The proposed work is designed to measure the production of neutron-induced DNA damage (single strand breaks), and the cells' ability to repair this damage under a variety of experimental conditions. The results with fast neutrons will be contrasted with those obtained with 250 kvp X-rays. Parallel experiments in which cell survival (viability) is measured will allow comparison of effects at the cellular and molecular levels. Data will be obtained as a function of cell type and cell age in the intermitotic cycle. The influence of energy metabolism will be studied in order to gain further insight into the biochemical mechanisms involved in DNA repair. Combined treatments with different modalities (e.g. neutrons plus the hypoxic cell sensitizer, metronidazole) will also be evaluated, both from the standpoint of cell survival, and for their effect on DNA repair. The results of this study will provide valuable information about the cellular and molecular (DNA) effects of fast neutrons on mammalian cells. The data will yield estimates of the RBE and OER for fast neutrons using both molecular and cellular endpoints. The influence of various conditions such as cell type, cell age in the mitotic cycle, hypoxia, and treatment with pharmacologic radiation modifiers, on the ability of neutron-irradiated cells to repair damage to their DNA, will provide more insight into the mechanism of neutron-induced cell inactivation.